[Bug libstdc++/29286] [4.0/4.1/4.2/4.3 Regression] placement new does not change the dynamic type as it should

["gdr at cs dot tamu dot edu" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #151 from gdr at cs dot tamu dot edu  2007-05-24 00:58 -------
Subject: Re:  [4.0/4.1/4.2/4.3 Regression] placement new does not change the
dynamic type as it should

"rguenther at suse dot de" <gcc-bugzilla@gcc.gnu.org> writes:

[...]

| > Gaby's model says that we know less about dynamic types than we
| > presently think we do, because there might be a union out there
| > somewhere.  (Fortunately, as Joseph points out, C99 has already answered
| > this question.  Surely we can agree that making C99 and C++ different in
| > this respect is a bad idea.)
| 
| I don't think dragging in unions helps us here ;)  Maybe Gaby can clarify
| if and how unions relate here, but I didn't percieve any previous comment
| as making implicit unions relevant here apart from what GCC and
| apperantly C99 agree to.

I believe we all agree that placement new changes the dynamic type.


I brought in the union example to point of a fundamental problem with
this issue.  I have been following the discussion without saying much,
until I realized that the interpretation Mark was offering is a
redefinition of the C++ object model that conflicts with the current
standard text. That was the point of the union example.  In the
example 

    void f(int* p, double* q) {
        *p = 42;
        *q = 3.12;
    }

All we know is that after the store to *p, the object there will have
type int (if it did not already have one).  Similarly, for the store
to *q, the object there will have type double.  Can the stores be
rearranged?  Under the current C++ rules (which were inherited from
C90, and not C99) "yes" if we know that the objects are distinct.
Can we infer the disjoinctness from the types?  "Not always" under
current C++ rules for union, and in this specific case, the answer is 
"no".

I never said I liked that model.  I was merely pointing out that
people where on the slope of redefining the object model.

I spent the afternoon trying to see how C++ can move forward.
The "effective types" of C99 has its own sets of incompleteness and
inconsistency problems that Nick MacLaren has brought to my attention
since I raised the issue on the C++ -core reflector.

-- Gaby


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29286